Electrical potentials can occur in a rotating shaft of turbines, generators, and motors due to a variety of reasons. Potentials can occur due to electrostatic charge buildup from steam or water contact with shaft components and capacitive or inductive coupling from dissymmetries in the stator or field windings, excitation system or equipment problems such as shorts in rotating field components or grounded bearing pedestals in the case on insulated bearings. Electrical potentials on a turbine/generator shaft must be grounded to avoid the voltage building up to high levels that cause problems such as arcs discharging across the oil film in bearings causing damage to surfaces in the bearings and resulting consequential damage. Shaft grounding systems typically connect to the shaft via carbon brushes or copper braids that rub against the rotating shaft. However, a semiconducting film due to dirt and oil can form between the brushes and shaft, reducing grounding effectiveness and allowing the shaft voltage to increase. Routine maintenance is required to clean the shaft and brushes. If maintenance is not performed on time, a poorly grounded shaft can cause a bearing or other component failure and a forced outage. The failure can cascade into consequential damage to seals, other bearings, and turbine blade tips. Damage to hydrogen seals in a generator can result in a hydrogen explosion. Other problems, such as shorts in insulated bearing pedestals or rotor windings can instantly result in very high shaft current, and cause a major outage well short of any maintenance interval.
U.S. Pat. No. 7,034,706 (Nippes) describes sampling the shaft voltage waveform, but only teaches standard electrical signal processing such as average and peak voltage limits and does not evaluate the shape of the waveform. The simple voltage average and peak values may then be used with other operational parameter alarm conditions, such as high bearing temperature, with Boolean logic. This does not make any evaluation of the waveform shape, which is a key indicator of potential problems.
U.S. Pat. No. 9,091,732 (Higgins) describes a shaft monitoring system that notifies a user of a fault condition based upon harmonic content of a voltage signal and electromagnetic interference (EMI) data.
Manual analysis by a human expert with an oscilloscope can analyze the waveform shape. However few people can perform such analysis at a high level of expertise, their time is expensive, and they are constantly learning by experience in the field. This experience is not instantly transferred to other experts. A human expert will typically only evaluate a turbine/generator at the time they are capturing data, so most of the time the system is unmonitored by a waveform shape expert, and a fault or predictive shape change can occur without notice.
U.S. Pat. No. 4,873,512 (Miller) is incorporated herein by reference in its entirety. It describes an active shaft grounding and diagnostic system that provides a counteracting voltage to the shaft of rotating equipment to minimize shaft voltage. Further improvement is needed; to monitor, diagnose, and predict faults; and to identify causes of faults.